Devices for detecting and transmitting at least one measurement signal characterizing the state of a vehicle tire, such as the tire pressure and/or the tire air temperature, are, in particular, tire pressure monitoring systems (TPMS) and are used to monitor the tire pressure in vehicles in order to prevent tire defects owing to an excessively low tire pressure, and therefore to reduce the number of accidents which are due to defective tires.
If a vehicle is operated with an excessively low tire pressure, this leads to increased flexing energy at the tire edges and therefore to increased wear of the tire. At high accelerations, a tire which is weakened in this way can, under certain circumstances, no longer stand the load and burst. In addition to damage to the tire, creeping tire collapse owing to the gas diffusion through the rubber of the tire over time is a significant cause of excessively low pressure in tires.
However, the tire inflation pressure is not only an important variable for road safety. The driving comfort, service life of the tire and fuel consumption are also significantly influenced by the inflation pressure. An inflation pressure which is reduced by 0.6 bar can increase the fuel consumption by up to 4% and shorten the service life of the tire by up to 50%.
The increasing number of tires with run-flat properties also requires the use of tire pressure monitoring systems since a car driver can no longer detect a tire with considerable underpressure on the basis of the driving behavior. In order to prevent the driver unknowingly exceeding the speed limits and distance limits which are valid for such a case, run-flat tires are allowed to be used only in conjunction with tire pressure monitoring systems or flat rolling warning systems.
Basically, two types of tire pressure monitoring system are differentiated: direct and indirect systems.
In direct systems, a sensor module with a pressure sensor is installed in each tire of the vehicle. Said sensor module transmits data from the interior of the tire, such as the tire pressure and the tire air temperature, to a control unit over an encoded radiofrequency transmission link. These data can be evaluated in the control unit and therefore, in addition to pressure losses in individual tires, slow pressure losses in all the tires (for example owing to diffusion through the rubber) are also detected. If the tire pressure drops below a defined threshold or if the pressure gradient exceeds a specific value, the driver is warned by a visual or acoustic signal. The sensor modules are generally supplied by a battery. This gives rise to additional requirements in terms of power consumption, resistance to media and acceleration sensitivity compared to other applications. Micro-mechanical absolute pressure sensors are used as the sensor elements.
The data which are measured with pressure and temperature sensor in the tire are conditioned in the sensor module, modulated onto an RF carrier signal and emitted via an antenna. This signal is either detected by individual antennas on the wheel cases or in a central receiver (for example in the control unit of existing remote keyless entry systems).
In indirect systems, a pressure loss in the tire is not determined directly but rather by a derived variable. For this purpose, a mathematical-statistical evaluation of the differences in the speeds of all the wheels from one another is carried out.
The wheel speed which is required for this is determined in vehicles with ABS systems by sensors which are already present, and said wheel speed is transferred to the ABS control unit. Differences in wheel speed occur if the diameter of the corresponding tire is reduced due to pressure loss, and therefore its wheel speed rises in relation to the other three tires. By forming differences, which can be carried out by cost-effective expansion of the ABS software algorithms, relatively large pressure losses on up to three tires can be detected.
Disadvantages of the indirect system are comparatively long detection time periods (10 mins) and a high detection threshold compared to direct systems. Current indirect systems which are commercially available are not capable of detecting slow pressure losses in all four tires due to diffusion. A further disadvantage of direct systems over indirect systems is the significantly higher costs and the battery-dependent, limited service life.
Depending on the type of activation of the transmission of data from the sensor module to the control unit, three systems are essentially known in the sensor modules. Continuously transmitting sensor modules; sensor modules which transmit only in the driving state of the vehicle, and sensor modules which transmit only when they are triggered by an LF trigger, for example.
Permanently transmitting sensor modules transmit their measurement signals to the control unit at fixed time intervals. Sensor modules which transmit only in the driving state of the vehicle generally have acceleration sensors by which the driving state can be detected as a function of the measurement signal.
These types of sensor modules are in the not-yet-installed delivery state in a current-saving standby mode, from which they first have to be changed into the operating mode. For this purpose, the sensor modules must be registered with the control unit and assigned to a vehicle tire.
A device of the generic type is discussed, for example, in German patent document DE 103 01 192 A1. A problem with the sensor modules of such devices is that hitherto their functional capability has been tested by a special RF communication module, which also includes checking the charge state of the electric energy store. However, this requires additional investment on the part of the vehicle workshops.
Furthermore, the sensor modules can also be installed without being tested. However, if it becomes apparent that the energy store in one sensor module or in some of the sensor modules installed on the vehicle is empty or defective, these sensor modules which have already been installed in or on the vehicle tire have to be removed again and replaced with functionally capable sensor modules, which entails a certain degree of expenditure.
The exemplary embodiments and/or exemplary methods of the present invention is therefore based on the object of further developing a device of the type mentioned at the beginning in such a way that simpler functional checking of the sensor modules is possible.
This object may be achieved according to the exemplary embodiments and/or exemplary methods of the present invention by the features described herein.